Wireless networks have become quite popular, but higher throughput wireless networks are constantly in demand. The throughput of a wireless network is a function of the bandwidth allocated to the wireless network and the data rates used, but the throughput also depends on the overhead, i.e., the proportion of the available bits that are allocated to signaling and control functions as opposed to conveying data between nodes of the network. If a fixed number of bits are available over a unit of time, if fewer of the available bits are consumed by signaling and control, then more bits are available for data transfer.
There are certain signals that are required in an efficient wireless network, such as signals used to convey to some stations that other stations are reserving the medium, to convey the existence of stations, and the like.
FIG. 1 illustrates the basic structure of conventional PPDU (PLCP Protocol Data Unit) frames. These frames all depend on and include a PLCP (Physical-Layer Convergence Procedure) preamble and a PLCP header followed by a PSDU (PLCP Service Data Unit). Where the PSDU is a control frame, it is typically sent at a basic service set rate, such as 1 Mbps (megabit-per-second) or 6 Mbps, so that all stations can be expected to hear and understand these frames.
A station will receive signals and process them in layers, with the station's physical (PHY) layer receiving the signal from the wireless medium, processing it and passing up the results of the processing to the station's media access control (MAC) layer, which in turn performs MAC processing and passes the results up to the next layer and so on. When the PHY layer gets the PPDU, it processes it to determine what bits were sent, corrects for correctable errors, then discards the PPDU if the errors are uncorrectable (and may also trigger error processing) or sends up the payload of the PPDU to the MAC (which is the PSDU contents, in this case). The MAC layer then decodes the PSDU and interprets its contents, such as by triggering control actions specified by the contents of a PSDU control frame.
Examples of PSDU control frames include frames used for an RTS-CTS (“Ready-To-Send/Clear-To-Send”) handshake. In an RTS-CTS handshake, one station might send an RTS message to an access point (or other coordinator or receiver) and the access point responds with a CTS message. All other stations hearing the CTS message will then know not to use the wireless medium, to avoid interfering with the station that sent the RTS message. If a station cannot hear and understand the CTS message (the hidden node problem), it might not know to defer and might interfere with a transmission. Because of this potential problem, as many stations as possible need to be able to receive and interpret these frames, so it would not be ideal to send them at high data rates that only some stations can receive. However, because of the low data rate, these frames take quite a lot of time to transmit, even though they are small frames. The time spent transmitting those frames is pure overhead, because no effective data transfer can happen during that time. Thus, if these frames could be made more efficient, data throughput would improve.